On my way home today I took this picture of MI6. We are based in MI9 next door so I regularly walk or cycle passed.
With this shot I tried to get the angle that is used in "The world is not enough". Bond bursts through the wall in Q's Jet Boat and chases the Cigar girl down the Thames.

The building, which opened in 1994, includes a shooting range and arsenal, a garage for adapting and maintaining MI6's fleet of special cars, secure areas where eavesdropping is impossible, and a array of computer and radio communication rooms. There are rooms for the development of specialist espionage equipment.

Mi6 was designed by Sir Terry Farrell, who was unaware of the intended tenants.The MI6 headquarters replicates Art Deco's distinctive symmetry and detail. The MI6 building is known to some staff as "Ceausescu Towers", after the former Romanian dictator with a penchant for grandiose and inhuman architecture.

The Secret Intelligence Service (SIS), commonly known as MI6 (see MI numbers) is the United Kingdom's external intelligence agency. Under the direction of the Joint Intelligence Committee (JIC), it works alongside the Security Service (MI5), Government Communications Headquarters (GCHQ) and the Defence Intelligence Staff (DIS). SIS is responsible for the United Kingdom's espionage activities overseas.

In 2001 the IRA fired a missile into the building which managed to break a window on the 8th floor.

From wiki the M directory is as follows:

MI1: Code-breaking (see GCHQ)

MI2: Russia and Scandinavia

MI3: Eastern Europe

MI4: Aerial Reconnaissance

MI5: Domestic intelligence (now Security Service)

MI6: Foreign intelligence (now Secret Intelligence Service)

MI7: Propaganda

MI8: Military Communication Interception (see GCHQ)

MI9: Undercover operations

MI10: Weapons analysis

MI11: Field security police

MI12: Military censorship

MI13: Reconnaissance

MI14 and MI15: German specialists

MI16: Scientific Intelligence

MI17: Military Intelligence "Head Office"

MI18: Unknown (may have never existed) (reason to believe North America)